1. Field
The present disclosure relates to a tire with a radial carcass reinforcement, and more particularly a tire intended for fitting to vehicles carrying heavy loads and travelling at sustained speeds, such as lorries, tractors, trailers or buses, for example.
2. Description of Relate Art
In the tires of heavy goods vehicles, the carcass reinforcement is generally fixed on either side in the area of the bead and is surmounted radially by a crown reinforcement composed of at least two layers, superimposed and formed of threads or cords which are parallel in each layer and crossed from one layer to the next making angles in the range between 10° and 45° with the circumferential direction. The said working layers forming the working reinforcement may be further covered by at least one layer, called the protective layer, formed by reinforcing elements which are advantageously metallic and extensible and are called elastic. It may also comprise a layer of metal threads or cords having low extensibility, forming an angle in the range from 45° to 90° with the circumferential direction, this ply, called the triangulation ply, being radially located between the carcass reinforcement and the first crown ply, referred to as the working ply, formed by parallel threads or cords lying at angles not exceeding 45° in absolute value. The triangulation ply forms a triangulated reinforcement with at least the said working ply, this reinforcement having low deformation under the various stresses which it undergoes, the triangulation ply essentially serving to absorb the transverse compressive forces acting on all the reinforcing elements in the crown area of the tire.
In the case of tires for “heavy-duty” vehicles, just one protective layer is usually present and its protective elements are, in the majority of cases, oriented in the same direction and with the same angle in absolute value as those of the reinforcing elements of the radially outermost and thus radially adjacent working layer. In the case of construction plant tires intended for running on more or less undulating ground, the presence of two protective layers is advantageous, the reinforcing elements being crossed from one layer to the next and the reinforcing elements of the radially internal protective layer being crossed with the inextensible reinforcing elements of the radially external working layer adjacent to the said radially internal protective layer.
Radially on the outside of the crown reinforcement is the tread usually made up of polymeric materials intended to come into contact with the ground in the contact patch in which the tire makes contact with the ground.
Cords are said to be inextensible when the said cords, under a tensile force equal to 10% of the breaking force, exhibit a relative elongation of at most 0.2%.
Cords are said to be elastic when the said cords exhibit, under a tensile force equal to the breaking load, a relative elongation at least equal to 3% with a maximum tangent modulus of less than 150 GPa.
The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the direction in which the tire runs.
The axis of rotation of the tire is the axis about which it turns in normal use.
A radial or meridian plane is a plane containing the axis of rotation of the tire.
The circumferential mid-plane, or equatorial plane, is a plane which is perpendicular to the axis of rotation of the tire and divides the tire into two halves.
The transverse or axial direction of the tire is parallel to the axis of rotation of the tire. An axial distance is measured in the axial direction. The expression “axially on the inside of or axially on the outside of, respectively” means “of which the axial distance, measured from the equatorial plane, is respectively less than or greater than”.
The radial direction is a direction that intersects the axis of rotation of the tire and is perpendicular thereto. A radial distance is measured in the radial direction. The expression “radially on the inside of or radially on the outside of, respectively” means “of which the radial distance, measured from the axis of rotation of the tire, is respectively less than or greater than”.
Certain present-day tires, referred to as “road tires”, are intended to run at high speed and over increasingly long journeys, because of improvements to the road network and the growth of motorway networks worldwide. Unquestionably, the set of conditions in which a tire of this type is required to run enables the distance covered to be increased because there is less tire wear; however, the endurance of the tire, and particularly that of the crown reinforcement, is adversely affected.
This is because stresses are present in the crown reinforcement; more particularly, there are shear stresses between the crown layers, combined with a significant rise in the operating temperature at the ends of the axially shortest crown layer, resulting in the appearance and propagation of cracks in the rubber at the said ends. This problem exists in the case of edges of two layers of reinforcing elements, the said layers not necessarily being radially adjacent.
In order to limit excessive temperature increases in the crown of the tire, the materials of which the tread is made are advantageously chosen to have hysteresis losses suited to the operating conditions of the tire.
Moreover, in order to improve the endurance of the crown reinforcement of the type of tire being studied, solutions relating to the structure and quality of the layers and/or profiled elements of rubber compounds which are positioned between and/or around the ends of plies and, more particularly, the ends of the axially shortest ply, have already been provided.
In order to improve the resistance to degradation of rubber compounds located in the vicinity of the edges of the crown reinforcement, patent FR 1 389 428 recommends the use, in combination with a low-hysteresis tread, of a rubber profiled element covering at least the sides and marginal edges of the crown reinforcement and composed of a rubber compound with low hysteresis.
In order to avoid separations between crown reinforcement plies, patent FR 2 222 232 teaches the coating of the reinforcement ends with a pad of rubber whose Shore A hardness is different from that of the tread surmounting the said reinforcement, and greater than the Shore A hardness of the profiled element of rubber compound placed between the edges of the crown reinforcement plies and the carcass reinforcement.
French application FR 2 728 510 proposes arranging, on the one hand between the carcass reinforcement and the crown reinforcement working ply radially closest to the axis of rotation an axially continuous ply formed of inextensible metal cords making with the circumferential direction an angle at least equal to 60° and of which the axial width is at least equal to the axial width of the shortest working crown ply and, on the other hand, between the two working crown plies an additional ply formed of metallic elements oriented substantially parallel to the circumferential direction.
French application WO 99/24269 further proposes, on each side of the equatorial plane and in the immediate axial continuation of the additional ply of reinforcing elements substantially parallel to the circumferential direction, that the two working crown plies formed of reinforcing elements crossed from one ply to the next be coupled over a certain axial distance and then uncoupled using profiled elements of rubber compound over at least the remainder of the width that the said two crown plies have in common.
This improvement in the endurance of tires means that the possibility of retreading when the tread has worn away can at least be contemplated. Specifically, where there is a desire to retread the tire after the tread has worn away, in order to optimize the use of the new tread the tire that is to be retreaded must not be in too advanced a state of ageing.
In order to increase the life of the tires still further, it is common practice to choose polymeric materials that have improved wear resistance properties from which to make the tread. Because such materials usually have an adverse effect on hysteresis properties it is also known practice to make the tread of a tire from a radial superposition of two different materials in order to obtain a wearing properties-hysteresis compromise that is satisfactory for the applications envisaged.
Such tires are, for example, described in document U.S. Pat. No. 6,247,512. That document describes the superposition of two layers of materials to form the tread, the external material coming into contact with the ground being notably better performing in terms of wear whereas the internal material has hysteresis properties that allow the increases in temperature of the tire in the crown region to be limited.
It is also known practice to modify the internal layer of such treads in order to combat irregular wear of the said tread that occurs notably after the first layer that forms the exterior surface of the tread of the new tire has worn away. Such internal layers of the tread may thus comprise axially outer parts made up of more rigid materials. Such tires are, for example, described in patent applications JP2007/196864 and JP2009/286317.
However, it is found that such treads are less favorable in terms of the hysteresis properties of the tire and therefore in terms of the properties of endurance and rolling resistance.